Requesting/granting in-app external storage permissions for devices Marshmallow (6.0/API 23) and above for a sample music player app (https://code.tutsplus.com/tutorials/create-a-music-player-on-android-project-setup--mobile-22764)

ext_strge_prmns_sample.txt

/**
  * Code snippet for requesting/granting permissions properly for dangerous permissions
  * such as external storage for devices M (6.0/API 23) and above. For sampling purposes,
  * snippet includes only the onCreate() and onRequestPermissionsResult() methods within
  * MainActivity.
 */
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // Requests permission for devices with versions Marshmallow (M)/API 23 or above.
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
        if (checkSelfPermission(Manifest.permission.READ_EXTERNAL_STORAGE)
                != PackageManager.PERMISSION_GRANTED) {

            requestPermissions(new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                    MY_PERMISSION_CODE);

            return;
        }
    }

    /**
     * The following code either executes for versions older than M, or until the user 
     * initially accepts the in-app permission for the next sessions.
     */
    init(); // Instantiating method/setup UI.
}

// Displays a permission dialog when requested for devices M and above.
@Override
public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
    if (requestCode == MY_PERMISSION_CODE) {
        
        // User accepts the permission(s).
        if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {

            // Instantiating method/setup UI.
            init();
            
        // User denies the permission(s).
        } else {
            Toast.makeText(this, "Please grant the permissions for Music Player 2.0 and come" +
                    " back again soon!", Toast.LENGTH_SHORT).show();

            // Runs a thread for a slight delay prior to shutting down the app.
            Thread mthread = new Thread() {
                @Override
                public void run() {
                    try {
                        sleep(1500);
                        System.exit(0);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            };

            mthread.start();
        }
    }
}

private void init() {
    songList = new ArrayList<Song>();
    songView = (ListView) findViewById(R.id.song_list);
    
    // Invokes the iteration for adding songs.
    getSongList();

    // Sorts the data so that the song titles are presented alphabetically.
    Collections.sort(songList, new Comparator<Song>(){
        public int compare(Song a, Song b){
                return a.getTitle().compareTo(b.getTitle());
            }
        });

        // Custom-base adapter instantiation that displays the songs via the ListView.
        SongAdapter songAdapter = new SongAdapter(this, songList);
        songView.setAdapter(songAdapter);

        // Initially passes the song list when the user first accepts the permission.
        mMusicService.setList(songList);

        // Sets the music controller up.
        setController();
}